Variable resistance device



July 7, 1942. o. v. HADLEY 2,289,115

VARIABLE RESISTANCE DEVICE Filed Dec. 4, 1940 INVENTOR 05cm 1 Had ZeyATTORNEY Patented July 7, 1942 VARIABLE RESISTANCE DEVICE Oscar V.Hadley, Plainfleld, ImL, assignmto P. R.

Mallory & 00., Inc., Indianapolis, Ind., a corporation of DelawareApplication December 4, 1940, Serial No. 368,435

3 Claims.

This invention relates to variable resistance devices such as volumecontrolsadapted for use in radio sets and the like.

An object of the invention is to improve variable resistance devices ofthe type described.

Other objects of the invention will be apparent from the followingdescription and accompanying drawing taken in connection with theappended claims.

The invention comprises the features of construction, combination ofelements, arrangement of parts, and methods of manufacture and operationreferred to above or which will be brought out and exemplified in thedisclosure hereinafter set forth, including the illustrations in thedrawlllg.

In the drawing:

Figure 1 is a section through a variable resistance device embodyingfeatures of the present invention, the section being taken on the line I-l of Figure 2;

Figure 2 is an end view of the device as seen from the shaft end;

Figure 3 is a section on the line 3-3 of Figure 1; and

Figure 4 is a perspective view of two cooperating parts of the device.

A feature of the present invention resides in the improvement in themechanical construction of the variable resistance device described. Oneof these mechanical improvements comprises the cooperation between ametal stiffener plate and an insulating sheet used for supporting themovable arm of the device. Another feature resides in the constructionof the movable arm. A further feature resides in the cover structure.Other features will be obvious from the following disclosure.

While a preferred embodiment of the invention is described herein, it iscontemplated that considerable variation may be made in the meth- 0d ofprocedure and the construction of parts without departing from thespirit of the invention. In the following description and in the claims,parts will be identified by specific names for convenience, but they areintended to be as generic in their application to similar parts as theart will permit.

Referring to the drawing, the variable resistance device comprises aninsulating base plate In of Bakelite or other sheet insulating materialupon which is supported a flat ring H of insulating material such aspaper, Bakelite fibre or other sheet insulating material having asurface l2 thereon of carbon paint or other resistance material. Baseplate I0 is provided with a central aperture IS. A metal bushing [4having a central aperture therein is clamped against the back of baseplate l0 by metallic shield plate IS with its aperture aligned with thecentral aperture in base plate l0.

A control shaft I6 extends through the aperture in bushing I4 and plateIn into the inside of the variable resistance unit. A clinch ring l'lfitting within a groove in shaft l6 limits the movement along the axisof shaft l8 into the bushing. The inner end of shaft is is provided witha reduced flattened end portion l8 and threaded thereover are, first, aninsulating sheet 20 of Bakelite or similar sheet insulating material,then a metal plate member 2| after which the end of reduced portion l8of the shaft is headed over to permanently secure the parts on the shaftand hold them together in rigid assembly. The apertures in sheet 20 andplate 2! are of similar cross-sectional shape to shaft position l8.

Ring ll and resistance surface l2 thereon have a short section cut outtherefrom so that the resistance surface does not form a complete circlebut has two terminal ends. These ends are preferably coated with ahighly conductive paint, such as silver paint, for providing goodterminal connections. Metal terminal lugs 22 and 23 are riveted to theback of base Ill and have connecting portions which pass throughapertures in base In and ring II, the inner ends of said connectingstrap portions are so preformed as to lay flat against the conductivepaint surface at the ends of the resistance element. A tap terminal 24is similarly riveted to the back of base plate l0 and has a connectingstrap portion passing up through an aperture therein and through anaperture in ring ll where its inner end is so formed as to lay downagainst a small arc of silver paint 25 paralleling the resistanceelement l2 on ring II. A silver paint bridge is applied at anappropriate point connecting the silver paint are 25 and the resistancepaint surface l2 at the point at which a tap terminal connection isdesired. From the shape of the terminal lugs 22, 23, and 24 it will beobvious that the lugs are preformed in the shape illustrated andinserted in the assembly with the inner ends contacting the silver paintsurfaces and then riveted in place, thereby applying a spring biasfirmly pressing the connecting strap end against the silver paintsurface.

An additional terminal 26 is mounted on base l0 midway between terminals22 and 23 and exbase l and ring II.

A contact arm 29 is riveted to insulating sheet 20 by rivet or eyelet 30and carries, at a diametrically opposite position from eyelet 38, acylindrical contact 3| which slides along resistancesurface l2. Contactarm 29 is made of spring metal, such as spring bronze, and is generallycircular in shape so that contact 9| is joined to the portion held byrivet 39 by two nearly semi-circular arms 4 going around opposite sidesof shaft l6. A pair of additional contact arms 32 integrally formed fromthe same sheet as arm 29 inside arm 29 ,extend around the two oppositesides of shaft l6 and have contacting end portions 33 which slide onring portion 21 of terminal 26. It will thus be seen that the contactarm element makes sliding contact both with the ring portion 21 ofterminal 26 and the resistance surface l2. Hence, terminal 26 comprisesthe contact arm terminal of the device.

Contact arm 29 is spring biased away from supporting insulating sheet 29and toward resistance surface I 2. Contact cylinder 3| is fitted withina generally rectangular aperture 34 in arm 29, the aperture havinginwardly extending projections at its two edges to provide'a rockingengagement with contact 3|. The end of arm 29 beyond contact 3| is bentdown at right angles as shown at 35 to properly cage contact 3| at thispoint and at the same time to decrease its length.

A feature of the present invention resides in the. means for stiffeningand supporting insulating sheet 2|) by the use of metal plate 2|. As isknown, materials such as Bakelite and other synthetic insulatingmaterials are subject to cold flow when placed under stress This resultsin a gradual'change in shape of the insulating support over periods oftime. Heretofore when contact arms for resistance devices have beenmounted on Bakelite arms the spring bias necessary to provide contactpressure has frequently produced suflicient cold flow in the Bakelitesupporting arm to result in an eventual change of shape of the arm withconsequent reduction of the contact pressure due to relaxing of thespring tension. Contact pressure in devices of the type described mustbe carefully controlled for proper operation of the device. If thepressure is too great the device is subjected to excessive wear andconsequent change in characteristics, while if the pressure is tooslight improper contact is made resulting in noisy and unstableoperation.

By providing stiffener plate 2| of the shape described cold fiow ofBakelite sheet 20 is effective- 1y prevented so that the contactpressure is held constant throughout the life of the device. It

I will be noted that the spring bias of arm 29 is such as to warp plate26 toward base |0 at the point where they are riveted together by eyelet90. To prevent this warping action the metal plate 2| is provided with agenerally rectangular aperture 36 and the portion 31 of plate 2| at theouter end of the aperture is offset to a position in a plane parallel tothe plane of the main portion of plate 2|. Bakelite sheet 20 is providedwith a tongue 38 on its edge beyond aperture 39 for receiving eyelet 39.Tongue 38 is. then threaded into aperture 86 of stiflener plate 2|before the parts are assembled on the reduced end of shaft l6. By thisconstruction the main body of insulating sheet 20 is covered with andstiffened by stiffener plate 2| while the tongue 28 is stifl'ened byportion 31 of plate 2| bearing against the opposite surface thereof. Itwill be seen that this effectively prevents deformation of sheet 20 bythe spring bias of arm 29.

A cylindrical metal casing 46 is provided for the operating parts of theresistance device. A pair of additional ears 49 are provided which canbe bent up, when desired, to serve as locating lugs for mounting thedevice in a radio set, the casing being secured to base III by cars 48bent around the edge of base l9 and shield plate IS. The end wall ofcasing 40 is provided with an opening 4| covered by a covered by a coverplate 42 secured thereon by hooks 43 which hook over shoulders formed inopposite edges of aperture 4| and spring tongue 44 which holds hookportions 42 in place. It will be obvious that cover plate 42 may readilybe removed by merely pressing spring tongue 44 thus permitting a snapacting AC switch to be secured to the back of casing.

Casing 40 is provided with a bent-in comer '45 and stiffener plate 2|has a projecting arm 46 which engages corner during its rotation therebylimiting the rotation of the control arm and preventing it fromoverrunning the ends of resistance surface l2. Stiflener plate 2| isalso provided with a small tongue 41 for engaging and operating theactuating lever of an AC switch when said switch is secured to the backof casing 40.

While the present invention, as to its objects and advantages, has beendescribed herein as carried out in specific embodiments thereof, it isnot desired to be limited thereby but it is intended to cover theinvention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. A variable resistance device comprising an arcuate plane resistancesurface, a shaft concentric therewith, and a contact arm assemblyclamped to said shaft, said assembly comprising a support of sheetinsulating material parallel to said resistance surface, a springcontact arm secured .tric point and joined to to said support at aneccentric point and extending around said shaft and engagingisaidresistance surface at a point diametrically opposite to said eccentricpoint, said arm being biased to press against said resistance surface,and thereby tending to bend said support out of its plane, and means toprevent said bending comprising a metal plate clamped against the outerface of said support, saidplate having an integral extension clampedagainst the inner face of said support in the region beyond saideccenthe main body of said plate by a pair of arms passing aroundopposite edges of said support.

2. A variable resistance device comprising an arcuate plane resistancesurface, a shaft concentric therewith, and a contact arm assemblyclamped to said shaft, said assembly comprising a support of sheetinsulating material parallel to said resistance surface, a springcontact arm secured to said support at an eccentric point and extendingaround said shaft and engaging said resistance surface at a pointdiametrically, opposite to said eccentric point, said arm being biasedto press against said resistance surface. and thereby tending to bendsaid support out of its plane, and means to prevent said bendingcomprising a metal plate clamped against the outer face of said support,said plate having an integral extension clamped against the inner faceof said support in the region beyond said eccentric point and Joined tothe main body of said plate by a pair of arms passing around oppositeedges of said support, said plate having an aperture between saidextension and the main body thereof between said arms to provide spacingbetween said plate and said spring contact.

3. A variable resistance device comprising an arcuate plane resistancesurface, a shaft concentric therewith, and a contact arm assemblyclamped to said shaft, said assembly comprising a support of sheetinsulating material parallel to said resistance surface, a springcontact arm secured to said support at an eccentric point and extendingaround said shaft and engaging said resistance surface at a pointdiametrically opposite to said eccentric point, said arm being biased topress against said resistance surface.

and thereby tending to bend said support out of its plane, and means toprevent said bending comprising a metal plate clamped against the outerface of said support, said plate having an integral extension clampedagainst the inner face of said support in the region beyond saideccentric point and Joined to the main body of said plate by a pair ofarms passing around opposite edges of said support, a case enclosingsaid resistance surface and contact arm assembly, said case having astop projection thereon, and said metal plate having a stop arm integraltherewith and positioned to encounter said stop projection in its rotarytravel.

. OSCAR V. HADLEY.

